The past 5 years has seen a new generation of analog and digital data measuring and analyzing equipment with ever-increasing performance requirements for their instrumentation grade laboratory tape transports. The impact of the integrated circuit has resulted in transducer preamplifier designs with lower drift, wider band width, better signal-to-noise ratios, higher common mode rejection, lower distortion, and the like.
Highly accurate A/D converters capable of generating data at rates of 80 .times. 10.sup.6 bits per second are available. Data conditioners, such as logarithmic compressors/expanders, high speed multiplexers, multilevel amplitude and phase modulators are used in many systems. Sophisticated real time wave analyzers permit examination of data down to 1/2 hertz band width and versatile computer programs allow detailed examination of data under different conditions.
In many instances, the data appears as short bursts of less than 1 minute duration but with frequency content of 4 mHz or higher. In other applications, the event may last many hours or days. In the former case, it is often necessary to expand a one second event over several minutes for proper analysis and in the latter case, economy may dictate that data gathered over several days be analyzed in a few hours. Often the analyzing is done at locations other than where the event took place.
In a number of these sophisticated applications the tape transports available today may often be the limiting factor in data analysis by reason of the introduction of errors in amplitude, time, and other factors, into the recorded data. Certain errors or limitations are imposed by the storage media (ferric oxide coated Mylar tape), but it is unlikely that the transport design has reached a state of excellence wherein the tape is the complete limiting factor.
There is a need for the improvement of many performance features not necessarily related to the tape, including (1) the provision of a recorder with greatly increased slew speeds for faster access to isolated pieces of data, (2) improved tape utilization by increasing the number of tracks per inch, (3) wider range of tape speeds for greater time-base expansion or compression, longer recording time and greater band width, (4) substantially lower velocity errors (flutter) for significant improvements in time-base errors, (5) precisely controlled tape tensioning under all modes of operation for reduction of tape damage (especially 0.5 mil. tape) and head wear, (6) improved skew characteristics for reduced interchannel time displacement errors, and (7) optimization of head and preamplifier designs for increased signal to noise ratios.
Tape recorders are generally identified as being of an open-loop configuration, a closed-loop configuration or a zero-loop configuration. In systems so designated, the loop is that section of the tape which passes the transducers under tension supplied by restraints at the respective ends. In the open-loop configuration, the tape is engaged by a capstan which performs the metering function and back tension is provided solely by the supply source. The length of tape between the take-up and supply reel is therefore unsupported, and there is little or no isolation of the portion of the tape at the transducers from the reel drives. Pinch rollers are frequently used in such systems, and in certain types of pinch roller installations, additional flutter and skew problems are introduced.
In the closed-loop configuration the section of tape passing the transducers is controlled by a metering element (usually through frictional contact at both ends of the passage). Thus, the metering element may be a single capstan, which the tape contacts twice, or, alternatively, may comprise two separate capstans which are mechanically connected. In the closed-loop tape system, there is a spring-mass by reason of the fact that the turn-around point for the tape is normally spatially separated by a relatively large distance from the capstan, and because of the spring-mass, the skew and flutter specifications are frequently less than desirable for the more demanding types of recording instruments.
In the zero-loop configuration the tape lays on the capstan and the heads are located in close proximity with the tape portion which is supported by the capstan. Such systems have the problem of tape head interface instability and excessive wear of the heads by reason of the tape/capstan contact with the head.